1. Field of the Invention
The invention generally relates to a permanent chill mold for the continuous casting of metals.
2. Description of Related Art
Tube-shaped chill molds made of copper or copper alloys, for casting profiles made of steel or other metals having a high melting point have been described many times in the related art. Permanent chill tubes usually have a uniform wall thickness in a horizontal cross sectional plane, which increases in the direction of the billet because of the inner conicity or taper of the chill tube. The conicity is able to be the same over the entire length of the permanent chill mold. However, conicities that are variable over the length may also be used, and the conicity may be greater especially in the region of the pouring slot, and may decrease in the casting direction, in order to be able to follow especially well the shrinkage of the cast billet in response to cooling, and thereby assure good heat removal.
Basically, measures for optimizing the conicity have the predominant aim of improving heat removal in the casting direction, by adapting the inside contour to the shrinkage of the strand shell. The majority of the permanent chill molds used these days is optimized to a certain operating point with regard to conicity, the operating point being a function of several parameters, such as casting speed, steel composition and the cooling conditions. When it comes to deviations from the predetermined operating point, the chosen geometry may lead to interference in the casting process and the billet quality, because on the billet, a so-called strand shell develops, as solidification of the molten metal in the casting bath level sets in. In the case of inappropriate permanent chill mold geometry of the chill tube, the strand shell may lift off and rotate, or, in the opposite case, that is, at too little shrinkage, this may lead to great friction at the chill tube. Bucking or jerking of the billet, billet spalling or even break-out may be the result. The air gap between the chill tube and the strand shell also brings about irregular heat removal, the strand shell melts again with the result of external and internal cracks in the billet. Therefore, there exists a multitude of efforts to adjust the conicity exactly to a certain application case, in order thereby to achieve optimum casting speeds.
In EP 0 958 871 A 1 it is proposed, for this purpose, to vary the conicity in at least a partial length of the casting cone, along a circumferential line, in such a way that each section of the circumferential line forms a smooth curve between the corner areas, with the conicity decreasing in the casting direction. Although this design of the mold cavity represents the theoretically optimum geometry for a certain set of parameters, in practice there will still be parameter fluctuations, conditioned, for example, upon temperature control or upon changed steel compositions which make it impossible durably to maintain the predetermined operating point of the permanent chill mold.